As illustrated in FIG. 13, a dump truck 1 as a construction machine is generally provided with a body frame 3, a pair of front wheels 6 rotatably arranged on opposite lateral ends of a front section of the body frame 3, respectively, a pair of rear wheels 7 rotatably arranged on opposite lateral ends of a rear section of the body frame 3, respectively, and a body 2 tiltably disposed on the body frame 3 and adapted to load thereon payload 9 such as earth, sand or crushed stone.
Specifically, the dump truck 1 is provided with hinge pins 5 arranged on the rear section of the body frame 3, and hoist cylinders 4 arranged on the body frame 3 at locations forward of the hinge pins 5 and connecting the body frame 3 and the body 2 with each other. Extension of the hoist cylinders 4 raises the body 2 to an elevated, in other words, “up” position, while retraction of the hoist cylinders 4 lowers the body 2 to a laid, in other words, “down” position while supporting the body 2. Therefore, the payload 9 such as earth, sand or crushed stone is loaded in a heap, for example, by a hydraulic excavator to a height above a payload space of the body 2 in its “down” position, the payload 9 loaded on the body 2 is transported to an intended location, and the body 2 is then brought from the “down” position into the “up” position so that the body 2 is tilted to unload the payload 9.
By loading the payload 9 on the body 2 as described above, the body 2 receives large loads at various parts thereof from the payload 9. The body 2 is, therefore, provided with a variety of stiffening members to prevent the body 2 from deformations or damage such as deflections under the weight of the payload 9. The body 2 has conventionally been constructed of a floor board 2a forming a floor, a front board 2b arranged on a front side of the floor board 2a and forming a front wall, side boards 2c arranged on opposite sides of the floor board 2a and forming side walls, and a canopy 2d arranged on an upper part of the front board 2b. Body structures for dump trucks, which include stiffener ribs and plates arranged as stiffening members, for example, on outer surfaces of these floor board 2a, front board 2b, side boards 2c and canopy 2d, are known (see, for example, Patent Documents 1 and 2).
As illustrated in FIG. 13, the stiffening members in the body structure for the dump truck in the conventional technology including Patent Documents 1 and 2 cited above include two frames 29a (see FIGS. 2 and 3), 29b, which are arranged on the floor board 2a, extend in a longitudinal direction of the body 2, and are placed with an interval therebetween such that they become laterally symmetrical to each other. When the payload 9 is loaded on the body 2 in the “down” position, the load of the payload 9 is applied to the entire floor board 2a, and therefore, the floor board 2a receives a downward force from the payload 9. As the floor board 2a receives at a central part thereof a normal force via the frames 29a, 29b at this time, moments occur at the opposite sides of the floor board 2a so that the floor board 2a tends to undergo a bending deformation in a downward direction. There is, accordingly, a need to secure rigidity sufficiently for the floor board 2a such that the above-mentioned bending deformation can be prevented.
In the conventional technology, the stiffening members in the body structure for the dump truck, therefore, include eight floor-board. stiffeners 41-48 arranged on the floor board 2a, which extend in the lateral direction of the body 2 and are placed with intervals in the longitudinal direction of the body 2. The rigidity of the entire floor board 2a is secured by these floor-board stiffeners 41-48 and frames 29a,29b. The stiffening members further include ribs 12, ribs 13, flat plates 14, flat plates 14a, flat plates 15, and flat plates 15a. The ribs 12 are arranged on and along upper edge portions of the side boards 2c. The ribs 13 are arranged extending in parallel with the corresponding ribs 12 on central parts of the side boards. 2c from a front of the body 2 toward a rear of the body 2. The flat plates 14 are arranged on front parts of the side boards 2c at locations between the ribs 12 and the ribs 13. The flat plates 14a are arranged to cover spaces defined by these side boards 2c, ribs 12, 13 and flat plates 14. Flat plates 15 are arranged on rear parts of the side boards 2c at locations between the ribs 12 and lower edge portions of the side boards 2c. The flat plates 15a are arranged to cover spaces defined by these side boards 2c, ribs 12, 13 and flat plates 15. By these ribs 12, 13 and flat plates 14, 14a, 15, 15a, the side boards 2c are provided with rigidity.
The stiffening members further include an unillustrated rib, and also, ribs 32a (see FIGS. 2 and 3), 32b. The unillustrated rib is arranged on a central part of the front board 2b, and extends in the lateral direction of the body 2. The ribs 32a, 32b are arranged on the central part of the front board 2b, extend in a vertical direction of the body 2 such that the ribs 32a, 32b intersect at right angles with the unillustrated rib, and are placed in parallel to each other with an interval therebetween in the lateral direction of the body 2. By these ribs, the front board 2b is provided with rigidity. The stiffening members further include ribs 19 arranged on and along opposite sides of the canopy 2d, and by these ribs 19, the canopy 2d is provided with rigidity to increase the strength of the canopy 2d of a cantilever structure. It is to be noted that as described above, FIG. 13 illustrates a left side of the dump truck 1 and neither the frame 29a nor the rib 32a is shown there.
The above-described ribs 12, 13, 19, 32a, 32b and floor-board stiffeners 41-48 and the unillustrated rib in the lateral direction of the front board are each formed to have a substantially “U” shaped cross section, and open end portions of these substantially “U” shaped cross sections are joined to the floor board 2a, front board 2b, side boards 2c and canopy 2d, respectively, by welding or the like, and the floor-board stiffeners 41-48 and frames 29a,29b for the floor board 2a are integrally joined to the floor board 2a by welding or the like. Further, the flat plates 14 for the side boards 2c are joined to the ribs 12, 13 by welding or the like, while the flat plates 14a are joined to the ribs 12, 13, flat plates 14 and side boards 2c by welding or the like. Upper portions of the front parts of the side boards 2c are formed in a box shape by the ribs 12, 13, flat plates 14, 14a and side boards 2c. Likewise, the flat plates 15 are joined to the ribs 12, 13 and side boards 2c by welding or the like, the flat plates 15a are joined to the ribs 12, 13, flat plates 15 and side boards 2c by welding or the like, and the rear parts of the side boards 2c are formed in a box shape by the ribs 12, 13, flat plates 15, 15a and side boards 2c. 